This invention relates to an electronic type electric energy meter based on a digital multiplication system.
FIG. 2 is a block diagram which schematically shows a prior-art electronic type electric energy meter described in the official gazette of Japanese Patent Application Publication No. 57-5466 by way of example.
Referring to the figure, numerals 1 and 2 designate input terminals to which the voltage and current of a circuit to-be-measured are respectively applied and which are respectively connected to the input terminals of A/D (analog-to-digital) converters 5 and 6. Numeral 7 designates a digital multiplier, the input terminals of which are connected to the output terminals of the A/D converters 5 and 6. On the other hand, the output terminal of the digital multiplier 7 is connected to the input terminal of an accumulator 8. The output terminal of the accumulator 8 is connected to the output terminal 9 of the electronic type electric energy meter 100.
Next, the operation of the electronic type electric energy meter 100 arranged as described above will be explained. A voltage and current proportional to the voltage and current of the circuit to-be-measured applied to the input terminals 1 and 2 are respectively supplied to the A/D converters 5 and 6. These A/D converters 5 and 6 A/D-convert the input voltage and current, and deliver digital values to the multiplier unit 7 respectively. The product of the digital values obtained by the multiplier unit 7 is a value which is proportional to the instantaneous power of the circuit to-be-measured, and which is accumulated by the accumulator 8. Thenceforth, such operations are repeated, and the accumulator 8 delivers a pulse output to the output terminal 9 each time the internal value thereof reaches a predetermined value.
With the prior-art electronic type electric energy meter 100, in order to satisfy the precision of an electronic type electric energy meter, an A/D converter of very high precision (for example, an A/D converter having many bits) is required of, for example, the A/D converter 6 on the current signal side. More specifically, the performance which is required of the electric energy meter is such that the precision is guaranteed for a voltage variation range of .+-.10% of a rated voltage value or so, but that the absolute precision is guaranteed for a current variation range of 1/60 to 1/1 of a rated current value. By way of example, when it is intended to guarantee the absolute precision of .+-.1% at the current value of 1/60 of the rating, a quantization error at this point needs to be less than 1/100, in other words, a resolution needs to be 1/128=1/.sub.2 7 which corresponds to 7 bits.
In this regard, since the current input becomes 60 times greater at the rated value, a dynamic range of at least 64 times=2.sup.6 times (corresponding to 6 bits) is needed. In consequence, an A/D converter of 7 bits+6 bits=13 bits is needed as the A/D converter 6. For the electronic type electric energy meter, a low cost and a high reliability are necessary conditions in view of the uses thereof. It is therefore an indispensable condition that an expedient for fabricating the meter is capable of mass production, nemely, monolithic IC implementation. However, an A/D converter having a high resolution of 13 bits is very difficult to attain using a monolithic IC, and presents serious hindrances to the lowering of power consumption, the reduction of cost and the mass production thereof.